U.S. patent number 5,415,715 [Application Number 08/068,738] was granted by the patent office on 1995-05-16 for method of manufacturing a honey comb structure of thermostructural composite material.
This patent grant is currently assigned to Societe Europeenne de Propulsion. Invention is credited to Andre Delage, Jean-Michel Georges, Jean-Pierre Maumus.
United States Patent |
5,415,715 |
Delage , et al. |
May 16, 1995 |
Method of manufacturing a honey comb structure of thermostructural
composite material
Abstract
A method of manufacturing a honeycomb structure of
thermostructural composite material comprising a fiber reinforcing
fabric densified by a matrix, the fibers of the reinforcing fabric
being of a material selected from carbon and ceramics, as is the
matrix, the method comprising the following steps: making a
three-dimensional fiber fabric by means of superposed
two-dimensional plies that are bonded together by means of fibers
passing through the plies; making slit-shaped cuts in a staggered
configuration through the plies, and through the entire thickness
of the fabric; stretching the cut fabric in a direction that is not
parallel to the cuts but that is parallel to the plies so as to
form cells whose walls are constituted by the lips of the cuts; and
while the cut texture is held in the stretched state, densifying it
using the matrix-constituting material to obtain a rigid honeycomb
structure of thermostructural material.
Inventors: |
Delage; Andre (Le Pian Medoc,
FR), Georges; Jean-Michel (Gradignan, FR),
Maumus; Jean-Pierre (Cenon, FR) |
Assignee: |
Societe Europeenne de
Propulsion (Suresnes, FR)
|
Family
ID: |
9430443 |
Appl.
No.: |
08/068,738 |
Filed: |
May 28, 1993 |
Foreign Application Priority Data
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Jun 4, 1992 [FR] |
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92 06790 |
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Current U.S.
Class: |
156/197; 156/229;
156/211; 428/118; 156/250; 428/116; 428/178 |
Current CPC
Class: |
B32B
3/12 (20130101); B31D 1/0031 (20130101); B32B
5/26 (20130101); E04C 2/365 (20130101); B32B
38/0012 (20130101); B29D 99/0089 (20130101); B32B
18/00 (20130101); B32B 38/00 (20130101); B32B
2305/024 (20130101); C04B 2237/76 (20130101); Y10T
156/1052 (20150115); Y10T 428/24165 (20150115); C04B
2237/765 (20130101); Y10T 428/24314 (20150115); B32B
2038/0028 (20130101); Y10T 156/1026 (20150115); Y10T
428/24661 (20150115); Y10T 156/1003 (20150115); C04B
2237/363 (20130101); C04B 2237/385 (20130101); Y10T
428/24149 (20150115) |
Current International
Class: |
B32B
18/00 (20060101); B31D 1/00 (20060101); B32B
3/12 (20060101); B29D 31/00 (20060101); E04C
2/34 (20060101); E04C 2/36 (20060101); B32B
031/18 () |
Field of
Search: |
;156/197,268,211,229,250
;428/116,118,178 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
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3729633 |
|
Mar 1989 |
|
DE |
|
WO9116277 |
|
Oct 1991 |
|
WO |
|
Other References
Database WPIL, Section Ch. Week 8925, Derwent Publications Ltd.,
London, GB, Class L02, AN 89-181962 & JP-A-1,119,574 (Showa
Hikoki Kogyo), May 11, 1989..
|
Primary Examiner: Ball; Michael W.
Assistant Examiner: Yoder; Michele K.
Attorney, Agent or Firm: Weingarten, Schurgin, Gagnebin
& Hayes
Claims
What is claimed is:
1. A method of manufacturing a honeycomb structure of
thermostructural composite material comprising a fiber reinforcing
fabric densified by a matrix, the fibers of the reinforcing fabric
and the matrix being of a material selected from the group
consisting of carbon and ceramics, the method comprising the steps
of:
making a three-dimensional fiber fabric (41) by superposing
two-dimensional plies (40) that are bonded together by fibers
passing through the plies;
making slit-shaped cuts (42) in a staggered configuration through
the plies and through the entire thickness of the fabric;
stretching the cut fabric in a single plane to form cells (46)
whose walls are constituted by the lips of the cuts (42); and
while the cut fabric is held in the stretched state, densifying it
using the matrix to obtain a rigid honeycomb structure (48) of
thermostructural material.
2. A method according to claim 1, characterized in that the
two-dimensional plies (40) of the three-dimensional fabric (41)
comprise layers of woven cloth.
3. A method according to claim 2, characterized in that the cuts
(42) are made parallel to one of the following directions: the warp
thread direction and the weft thread direction of the layers of
cloth.
4. A method according to claim 2, characterized in that the cuts
are made in a direction that is inclined relative to the warp
thread direction and to the weft thread direction of the layers of
cloth.
5. A method according to claim 1, characterized in that the
two-dimensional plies of the three-dimensional fabric comprise
single-directional sheets whose directions are inclined relative to
one another.
6. A method according to claim 1, characterized in that the bonding
between the plies (40) of the three-dimensional fabric is provided
by needling.
7. A method according to claim 1, characterized in that the cuts
(42) are made perpendicularly to the plies.
8. A method according to claim 1, characterized in that the cuts
are made in planes that are inclined relative to the normal to the
plies.
9. A method according to claim 1, characterized in that the cut and
stretched fabric (47) is densified by chemical vapor infiltration
while being held in the stretched state by means of a jig (50,
52).
10. A method according to claim 1, characterized in that the cut
and stretched fabric (47) is provided with at least one fiber layer
(54, 55) on each of its faces parallel to the plies of the fabric,
and the assembly formed by the stretched fabric and the fiber
layers is densified to obtain a panel (58) comprising a rigid
honeycomb core covered by a rigid skin on each face.
11. A method according to claim 10, characterized in that the fiber
layer placed on each face of the fabric is bonded thereto by
needling.
12. A method according to claim 1, characterized in that the cuts
(42) are made in a plane fabric (41).
13. A method according to claim 1, characterized in that the cuts
(62) are made in a cylindrical fabric (61).
14. A method according to claim 13, characterized in that the cuts
(62) are made in meridian planes and the fabric is stretched
circumferentially.
15. A method according to claim 13, characterized in that the cuts
are made in planes perpendicular to the axis of the fabric and the
fabric is stretched parallel to its axis.
16. A method according to claim 1, characterized in that the cut
fabric is given a conical shape.
17. A method of manufacturing a honeycomb structure of
thermostructural composite material comprising a fiber reinforcing
fabric densified by a matrix, the fibers of the reinforcing fabric
and the matrix being of a material selected from the group
consisting of carbon and ceramics, the method comprising the steps
of:
making a three-dimensional fiber fabric (41) by superposing
two-dimensional plies (40) that are bonded together by fibers
passing through the plies;
making slit-shaped cuts (42) in a staggered configuration through
the plies and through the entire thickness of the fabric;
stretching the cut fabric in a single plane to form cells (46)
whose walls are constituted by the lips of the cuts (42); and
while the cut fabric is held in the stretched state, densifying it
using the matrix to obtain a rigid honeycomb structure (48) of
thermostructural material;
the two-dimensional plies (40) of the three-dimensional fabric (41)
comprising one of: layers of woven cloth; or single-directional
sheets whose directions are inclined relative to one another;
the cuts (42) being one of: made parallel to the warp thread
direction of the layers of the cloth; made parallel to the weft
thread direction of the layers of cloth; made in a direction that
is inclined relative to the warp thread direction of the layers of
the cloth; or made in a direction that is inclined relative to the
weft thread direction of the layers of cloth;
the bonding between the plies (40) of the three-dimensional fabric
being provided by needling;
the cuts (42) being one of: made perpendicularly to the plies; or
made in planes that are inclined relative to the normal to the
plies;
the cut and stretched fabric (47) being densified by chemical vapor
infiltration while being held in the stretched state by means of a
jig (50, 52);
the cut and stretched fabric (47) being provided with at least one
fiber layer (54, 55) on each of its faces parallel to the plies of
the fabric, and the assembly formed by the stretched fabric and the
fiber layers being densified to obtain a panel (58) comprising a
rigid honeycomb core covered by a rigid skin on each face with the
fiber layer placed on each face of the fabric bonded thereby by
needling; and
the cuts (42) being made in one of: a plane fabric (41) and a
cylindrical fabric (61) with the cuts (62) one of: made in meridian
planes and the fabric stretched circumferentially; or made in
planes perpendicular to the axis of the fabric and the fabric is
stretched parallel to its axis.
Description
FIELD OF THE INVENTION
The present invention relates to the manufacture of honeycomb
structures of thermostructural composite material.
BACKGROUND OF THE INVENTION
Thermostructural composite materials are characterized by their
mechanical properties that make them suitable for constituting
structural elements and by their ability to conserve their
mechanical properties at high temperatures. Typical
thermostructural composite materials are carbon-carbon (C-C)
composites and ceramic matrix composite (CMC) materials.
C-C composites are constituted by a reinforcing texture or
"preform" of carbon fibers that is densified by a matrix of carbon.
CMCs are constituted by a preform of refractory fibers (carbon
fibers or ceramic fibers) densified by a ceramic matrix. A ceramic
material commonly used for manufacturing CMCs is silicon carbide
(SIC).
The preform of a C-C composite or of a CMC is made by stacking or
draping unidirectional plies (sheets of mutually parallel yarns or
cables) or multidirectional plies (pieces of woven cloth, webs of
fibers, pieces of felt), or by winding yarns, tapes, or strips, or
by three-dimensional weaving. When draping plies, they may be
bonded together by needling, by sewing, or by implanting transverse
threads. Preforms are made of ceramic or carbon fibers, or more
generally of fibers made of a precursor of ceramic or carbon, with
the precursor then being transformed after the textile operations
required for manufacturing the preform have been completed.
The purpose of densifying a preform is to fill the accessible pores
thereof with the matrix-forming material. Such densification can be
implemented by impregnating the preform with a liquid that contains
a precursor of the matrix material and then transforming the
precursor, or by chemical vapor infiltration.
The techniques mentioned above for making fiber preforms out of
carbon or ceramic, and for densifying them by means of a carbon
matrix or a ceramic matrix are well known.
Several methods exist for manufacturing honeycomb structures.
A first known method (FIGS. 1A, 1B, and 1C) consists in stacking
together sheets 10 and in gluing them together in a staggered
configuration. Gluing takes place along parallel strips 12 with
strips of glue situated on one face of a sheet being offset
relative to strips of glue situated on its other face (FIG. 1A).
The set of sheets is then cut up into slices 14 perpendicularly to
the strips of glue. Each slice is then stretched in the direction
normal to the faces of the sheets (arrows f in FIG. 1B) so as to
obtain hexagonal cells 16 by deformation (FIG. 1C). A honeycomb
panel 18 is then obtained, and metal or composite sheets may be
stuck on its opposite faces.
Such a method is used for making metal honeycomb structures. The
sheets 10 are cut out from sheet metal, and the cells 16 are
produced by plastic deformation of the metal.
That method can also be implemented using sheets of card or of
paper. In which case, after being stacked and glued in a staggered
configuration, the sheets of paper may be impregnated with a resin,
e.g. a phenolic resin. The resin is cured after the cells have been
formed (which happens either before or after the set of sheets has
been cut up into slices).
To make a honeycomb structure out of a thermostructural composite
material, it would be possible to implement a method of the same
type using two-dimensional fiber plies, e.g. plies of woven cloth
that are stacked and glued in a staggered configuration.
Densification and consequently rigidification of the structure
would then be performed after stretching and cell formation. Each
ply would normally be made up of a plurality of layers of cloth,
thus requiring the layers in a given ply to be bonded together in
order to prevent them separating during stretching. In addition, it
is difficult to perform a gluing operation in a staggered
configuration on cloth with the regularity and accuracy required
for ensuring that cloth is not torn because of a local defect when
stretching is applied. In addition, during the densification
operation after stretching, there is a danger of thermal stresses
rupturing the glue.
One solution would be to sew the plies of cloth together in a
staggered configuration, instead of gluing them together, but
although that would avoid certain drawbacks, it would also give
rise to considerable difficulties of implementation.
A second known method (FIGS. 2A, 2B) consists in using corrugated
sheets, e.g. of metal foil. The corrugated sheets 20 are superposed
and glued or welded or soldered together along their touching
facets 22 (FIG. 2A). Honeycomb panels 28 are obtained directly by
slicing the block of sheets 20 perpendicularly to the corrugations
(FIG. 2B).
That method can be used for making honeycomb structures out of
composite material by using corrugated sheets that are themselves
made of composite material. Such sheets can be obtained by draping
layers of cloth so as to give them the desired corrugated shape and
then densifying them, e.g. by draping and molding layers of cloth
that have been preimpregnated with a resin or with some other
liquid precursor for the matrix of the composite material, and then
applying heat treatment. A method of that type is described in
document WO 91/16277. However, it is then necessary to glue the
corrugated sheets together in a manner that is effective and
capable of withstanding the operating temperatures to which
thermostructural materials may be subjected in use. In addition,
the operations of prefabricating corrugated sheets are lengthy and
expensive, thereby considerably increasing the cost of the
honeycomb structure.
Finally, a third known method (FIGS. 3A and 3B) uses a sheet 30,
e.g. a metal sheet, in which cuts 32 are formed. The cuts are
formed in a staggered configuration along parallel lines (FIG. 3A).
The cuts are of equal length and they are regularly spaced apart
along each line. The cuts situated along one line are offset
relative to those in the adjacent lines, and each cut extends over
a length that is greater than the distance between two adjacent
cuts in the same line. The sheet 30 is expanded by opening the cuts
and forcing metal out of the plane of the sheet so as to form cells
36 at the locations of the cuts by plastic deformation of the metal
(FIG. 3B). The expansion is limited so as to avoid generating
stresses that could tear the sheet, particularly at the ends of the
cuts 32. The axis of each cell is inclined relative to the initial
plane of the sheet through an angle of less than 90.degree. such
that the walls of the cells are not perpendicular to the general
plane of the resulting honeycomb panel 38.
The expanded metal technique is practically impossible to transpose
to composite materials. They do not have the same capacity as metal
for plastic deformation. Expanding layers of cloth prior to
densification and rigidification runs a high risk of tearing the
cloth at the ends of the cuts, and gives rise to a problem of
holding the expanded cloth in shape. In addition, that method
suffers from a major limitation as to the thickness of the
honeycomb panel that can be obtained. This thickness is determined
by the distance between the lines of cuts, and it must be
sufficiently small to ensure that expansion can be achieved fairly
easily.
SUMMARY OF THE INVENTION
An object of the present invention is thus to provide a method
enabling a honeycomb structure to be made of thermo-structural
composite material without encountering the above drawbacks.
In particular, an object of the invention is to provide a method
whereby honeycomb structures of thermostructural composite material
can be made at a cost that is low enough to open up wide areas of
application.
This object is achieved by a method of manufacturing a honeycomb
structure of thermostructural composite material comprising a fiber
reinforcing fabric densified by a matrix, the fibers of the
reinforcing fabric being of a material selected from carbon and
ceramics, as is the matrix, the method comprising the following
steps:
making a three-dimensional fiber fabric by means of superposed
two-dimensional plies that are bonded together by means of fibers
passing through the plies;
making slit-shaped cuts in a staggered configuration through the
plies, and through the entire thickness of the fabric;
stretching the cut fabric in a direction that is not parallel to
the cuts but that is parallel to the plies so as to form cells
whose walls are constituted by the lips of the cuts; and
while the cut texture is held in the stretched state, densifying it
using the matrix-constituting material to obtain a rigid honeycomb
structure of thermostructural material.
The plies forming the reinforcing fabric may be constituted, at
least in part, by layers of cloth. Bonding between the plies is
achieved, for example, by implanting threads, by sewing, or by
needling. When needling plies that comprise layers of cloth, it may
be advantageous to interpose therebetween plies made of fiber webs
so as to provide a source of fibers suitable for being taken by the
needles to be placed transversely through the plies during
needling.
Fiber reinforcing fabrics formed by superposed two-dimensional
plies that are bonded together by needling, for example, are well
known.
The method of the invention is remarkable, in particular, in that a
preform for a honeycomb structure is obtained simply by forming
staggered cuts and by stretching the fabric.
This method differs from that shown in FIGS. 1A to 1C in that,
according to the invention, the stretching takes place parallel to
the planes of the plies and not perpendicularly thereto.
The method of the invention also differs from that shown in FIGS.
3A and 3B. With the prior method, the walls of the cells are
constituted by portions of the sheet situated between two lines of
cuts. Expansion has the effect of causing these walls to be
inclined relative to the initial plane of the sheet as the sheet is
made larger. This is not true of the method of the present
invention. The effect of stretching the fabric is to move apart the
lips of each cut so as to form cells whose walls are formed by the
lips of the cuts. The thickness of the honeycomb structure is
determined by the thickness of the fiber fabric and therefore does
not suffer from the same limitation as occurs in the method shown
in FIGS. 3A and 3B where the thickness of the honeycomb structure
is determined by the (necessarily limited) distance between pairs
of adjacent lines of cuts.
The invention also provides a honeycomb structure of
thermostructural composite material as can be obtained by the
above-defined method.
According to the invention, such a structure comprising a fiber
reinforcing fabric densified by a matrix is characterized in that
the reinforcing fabric is a three-dimensional fabric formed by
two-dimensional plies bonded together by fibers passing through the
plies, the cells of the honeycomb structure being formed through
the plies.
Other features and advantages of the method and of the structure of
the present invention appear on reading the following description
given by way of non-limiting indication.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is made to the accompanying drawings, in which:
FIGS. 1A, 1B, and 1C, described above, show a prior art method of
making a honeycomb structure;
FIGS. 2A and 2B described above, show another prior art method of
making a honeycomb structure;
FIGS. 3A and 3B, described above, show yet another prior art method
of making a honeycomb structure;
FIGS. 4A to 4F show various successive steps in one implementation
of the method of the invention for manufacturing a plane honeycomb
fabric of thermostructural composite material;
FIGS. 5A to 5C show how a skin can be formed on a honeycomb
structure to make a panel; and
FIGS. 6A and 6B show another implementation of the method of the
invention for making a cylindrical honeycomb structure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A method of the invention for making a plane honeycomb structure of
thermostructural composite material of the carbon/carbon type is
now described with reference to FIGS. 4A to 4F.
A first step in the method consists in making a three-dimensional
reinforcing fabric of carbon fibers.
To this end, two-dimensional plies 40 of carbon or of a carbon
precursor (e.g. polyacrylonitrile or "PAN" in the pre-oxidized
state) are superposed and needled together (FIG. 4A). The plies 40
are layers, e.g. of woven cloth or of a complex made up of cloth
and fiber web with the fiber web providing fibers that are easily
taken by the needles during needling so as to be implanted through
the plies. The plies 40 could also be constituted by
single-directional sheets, such as sheets of mutually parallel
cables or yams. In particular, each ply could be constituted by
three pre-needled single-directional sheets with the directions of
the three sheets being at angles of 60.degree. to one another.
Needling is preferably performed on fibers in the carbon-precursor
state, since needling performed directly on carbon fibers has a
more destructive effect. Needling may be performed progressively
while the stack of plies 40 is being built up, using the method
described in Japanese Patent Application filed under No 61-149880
on Jun. 27, 1986, with the thickness of the fabric being determined
as a function of the thickness of the honeycomb structure to be
made.
Other techniques may be used for bonding the plies together, e.g.
sewing, or else implanting threads as described in U.S. Pat. No.
4,628,846.
When the three-dimensional fabric 41 obtained in this way is made
of carbon-precursor fibers, carbonizing heat treatment must be
performed to transform the precursor into carbon. Since such
treatment gives rise to a small amount of shrinkage, it is
preferably performed before cuts or slits are made in the fabric
for the purpose of forming the cells in the honeycomb
structure.
As shown in FIG. 4B, these slit-shaped cuts 42 are made in a
staggered configuration, with the sizes and positions thereof
defining the sizes and shapes of the cells. The cuts 42 are made in
planes that are mutually parallel and perpendicular to the planes
of the plies 40.
The planes of the cuts may be parallel to one of the directions X
and Y along which the warp threads and the weft threads of the
cloth in the plies 40 extend, e.g. along the warp thread direction
X (with the layers of cloth being superposed so that their warp
threads are parallel, and consequently so that their weft threads
are also parallel). As a result, continuity of the warp threads (or
of the weft threads) is conserved in the plies of cloth after the
cuts have been made. However, this is not always necessary. The
planes of the cuts may intersect both directions X and Y and the
layers of cloth may be superposed so that their warp (and weft)
threads are not parallel, without damaging the strength of the cut
fabric nor its capacity for deformation.
In the example shown, the cuts all have the same length L and they
are regularly spaced apart, being separated by the same distance D
in each plane. The planes themselves are regularly spaced apart
with the distance between planes or pitch being d. The length L of
the cuts is greater than the length D of the gaps between the cuts
and the staggered disposition in the example shown is such that the
middle of a cut 42 in one plane of cuts is level with the middle of
the gap between two cuts 42 in the adjacent planes of cuts.
The cuts 42 may be made, for example, using a blade or a water
jet.
After the cuts have been made, the fabric 41 is stretched in the
direction perpendicular to the planes of the cuts (arrows F in FIG.
4C). Stretching causes the lips of the cuts 42 to move apart (FIG.
4C), thereby forming cells 46 whose walls are defined by the lips.
Stretching is stopped when the cells 46 have achieved the desired
shape (FIG. 4D) and before the stresses exerted at the ends of the
cuts cause the fabric to tear.
It may be observed that the fabric can be stretched in a direction
that is inclined relative to the planes of the cuts, i.e. in a
direction that is not necessarily perpendicular thereto.
A honeycomb fiber preform 47 is thus obtained in which the walls of
the cells 46 are perpendicular to the planes of the plies 40.
It is possible to make the cuts 42 in planes that are inclined
relative to the normal to the plies 40. After stretching, cells are
then obtained whose walls are not perpendicular to the faces of the
fabric.
Tests performed on fabrics as described above have shown that
during stretching the walls of the cells remain perpendicular to
the planes of the plies and that surface deformations, in
particular as occur in the end regions of the cuts, remain very
small in amplitude. It has also been observed that stretching does
not cause tearing at the ends of the cuts. By way of comparison,
tests performed on fabrics that are identical except for needling
(no bonding between plies) have shown that stretching can cause
damage to the fabric at the ends of the cuts.
It should be observed that the capacity of the fabric 41 for
deformation under traction parallel to the plies is surprising
insofar as cloth has the reputation of being undeformable in its
own plane.
After stretching, the preform 47 is densified while being held in
the stretched state by means of a jig. The jig (FIG. 4E) is
constituted by a graphite soleplate 50 and graphite pegs 52
extending into cells 46 along the edges of the preform and disposed
in opposite positions in the Y direction. The pegs 52 penetrate
into holes formed in the soleplate 50.
The assembly constituted by the jig 50, 52 and the preform 47 is
placed in an oven in which the preform 47 is densified by carbon by
means of chemical vapor infiltration. In conventional manner, a gas
containing one or more hydrocarbons is injected into the enclosure
under conditions of temperature and pressure that are determined to
facilitate decomposition of the gas on coming into contact with the
fibers of the preform 47, thereby releasing carbon which
progressively fills up the pores in the preform 47.
After densification, a honeycomb structure 48 of carbon/carbon
composite is obtained (FIG. 4F). There are numerous applications
for such a structure. For example, it may constitute a soleplate
for a heat treatment oven, advantageously replacing a metal
soleplate obtained by casting or by welding elements together. Such
a honeycomb structure may also be used as a rigid jig for holding a
preform that is to be densified by chemical vapor infiltration,
replacing a conventional graphite jig.
Honeycomb structures of thermostructural composite material have
other applications, e.g. in making up planes suitable for use in
aviation or space applications, e.g. as structural components of
space airplanes.
In some applications, the honeycomb structure may be provided with
a skin on each face.
To this end, as shown in FIG. 5A, at least one fiber layer 54, e.g.
a layer of cloth, is tensioned over the preform 47 while still held
on the soleplate 50 by means of the pegs 52. The layer of cloth 54
is needled to the edges of the cells 46 using a needling head whose
displacements can be programmed, e.g. as described in French Patent
Application No 2,669,941.
After the layer 54 has been needled to the cells, a graphite
soleplate 51 analogous to the soleplate 50 is placed on the preform
47 together with its layer of cloth 54, the soleplate 51 having
holes appropriately placed for receiving the top ends of the pegs
52 which stand proud of the preform 47. The assembly is turned
over, and the soleplate 50 is removed so as to allow at least one
other layer of cloth 55 to be tensioned over the other face of the
preform 47 and to enable said other layer to be needled in place
(FIG. 5B).
The assembly is then inserted in a chemical vapor infiltration
furnace so as to densify the preform 47 simultaneously with the
layers of cloth 54 and 55 needled to its two faces, thus making it
possible to obtain a panel 58 comprising a rigid honeycomb core 48
covered by two rigid skins 56 and 57 that close the cells 46 (FIG.
5C).
When the panel being made is not going to be subjected to large
shear forces, the layers of cloth 54 and 55 may merely be glued to
the faces of the preform 47 prior to being densified together
therewith, such that co-densification provides the additional
bonding required between the skin and the core of the panel.
In the above description, the composite material used for making
the honeycomb structure is a carbon/carbon composite.
However, the invention is naturally applicable to making honeycomb
structures of thermostructural composite materials other than
carbon/carbon composites, and in particular composites having a
ceramic matrix with a reinforcing structure that is made of carbon
or of ceramic. The techniques implemented are known techniques for
making three-dimensional fabrics of carbon fibers or of ceramic
fibers, and for densification by means of a ceramic matrix.
It may also be observed that a honeycomb preform optionally fitted
with fiber layers on its faces may be densified by a liquid method,
i.e. by being impregnated with a liquid precursor of the matrix,
and then transforming the precursor. Several impregnation cycles
may be necessary, optionally together with a chemical vapor
infiltration cycle.
Finally, although the honeycomb structures in the above description
are plane, the invention is applicable to making structures that
are curved, cylindrical, or even conical. Such structures may be
obtained by shaping the honeycomb preform on an appropriate jig,
prior to densification and rigidification. For a honeycomb
structure that is conical or frustoconical, the cuts may be formed
in the fiber fabric at a varying pitch such that after the fabric
has been stretched and shaped, the resulting cells are of
substantially the same size. The term "varying pitch" is used to
indicate that the distance between adjacent planes of cuts varies
from one side of the fabric to the opposite side.
It is also possible to make a cylindrical three-dimensional
reinforcing fabric 61 by needling together plies 60 that have been
wound on a mandrel (FIG. 6A) as described in Japanese Patent
Application ruled under No 61-149881 on Jun. 27, 1986, for example.
Cuts 62 are formed in a staggered configuration along meridian
planes through the entire thickness of the fabric 61.
The cut fabric is stretched onto a mandrel 70 of diameter that is
greater than that of the mandrel 60, thereby forming the cells 66.
Pegs 72 implanted in the mandrel 70 hold the fabric in its
stretched state for densification purposes, e.g. chemical vapor
infiltration (FIG. 6B). After densification, a rigid cylindrical
honeycomb structure is obtained.
FIG. 6A shows cuts formed in meridian planes. In a variant, it is
possible to form cuts in a staggered configuration in planes that
are perpendicular to the axis of the fiber fabric. Cells are then
obtained by stretching the fabric parallel to its axis.
* * * * *